Time-dependent kinetic complexities in cholinesterase-catalyzed reactions

Area of Science:

• Biochemistry
• Enzymology
• Chemical Kinetics

Background:

• Cholinesterases (ChEs) exhibit complex kinetic behavior, including hysteresis and damped oscillations, with certain substrates and inhibitors.
• These time-dependent phenomena are observed in both butyrylcholinesterase and acetylcholinesterase from various sources.

Purpose of the Study:

• To interpret the hysteretic behavior of cholinesterases in terms of slow transitions between enzyme conformers.
• To explore the role of substrate properties and enzyme type in determining reaction pathways and oscillations.
• To understand the mechanism of biphasic inhibition of ChEs by carbamates and organophosphates.

Main Methods:

• Kinetic analysis of cholinesterase reactions with various substrates and inhibitors.
• Investigation of the influence of medium parameters (pH, salts, solvents, pressure, temperature) on hysteresis.
• Review of mutagenesis and crystallographic studies related to ChE hysteresis.

Main Results:

• Hysteresis in ChE reactions can be explained by slow interconversion between two enzyme forms (E and E'), with different substrate binding and catalytic efficiencies.
• Damped oscillations arise when substrates exist in slowly interconverting forms, with only a minor form being reactive.
• Inhibition by carbamates and organophosphates fits a model of slow equilibrium between two reactive enzyme forms.
• Medium parameters, particularly water structure, modulate ChE hysteresis.
• Evidence suggests His438 conformation is critical for hysteresis, but a precise molecular mechanism remains elusive.

Conclusions:

• The slow conformational equilibrium between enzyme states (E and E') provides a framework for understanding ChE hysteresis and oscillations.
• His438 conformation is likely a key determinant of hysteretic behavior.
• ChE hysteresis may serve a regulatory function, damping responses to ligands and inhibitors, particularly for membrane-bound enzymes in crowded environments like synaptic clefts.
• The physiological and toxicological relevance of these hysteretic phenomena cannot be dismissed.